Monitoring systems for monitoring physical quantities or parameters are used in many applications such as manufacturing plants, chemical processing plants, pipe lines and telephone cables. The physical quantities monitored include fluid pressure, flow rate, temperature, humidity, etc. An important application is that of monitoring fluid pressure in telephone cables. This application typically requires the use of transducers at monitoring points which may be located up to one hundred miles or so from the monitoring or central station. In this kind of application, it is desired to use a large number of transducers with a minimum of transmission lines and power requirements while realizing a high degree of accuracy and reliability in the acquisition of data representing the values of the monitored physical quantities.
In the prior art, the Goldstine et al U.S. Pat. No. 3,327,289 discloses a remote monitoring system especially adapted for multiple transducers. The central station is connected through a pair of wires to multiple remote transducers each of which is selectively responsive to a tone frequency. An interrogator circuit at the central station selectively applies the tone frequencies to the pair of wires and a constant current source is continuously connected to the pair of wires. When a transducer receives the tone frequency assigned to it, a switching transistor is turned on and a variable resistor representing the physical quantity is connected across the pair of wires. Compensation is provided for the resistance of the wires and the voltage appearing on a resistor at the central station is proportional to the value of the quantity being monitored.
The Whiteside U.S. Pat. No. 4,103,337 discloses a monitoring system in which data is transmitted from a remote sensor using a form of analog to pulse width to digital conversion. A central processor transmits a data request signal on a single wire transmission line to the remote sensor. The data request signal activates the sensor and the interface electronics and also provides electrical power thereto. Upon receipt of the data request signal, the interface electronics generates a low impedance to ground signal on the transmission line after a time lapse corresponding to the value of the quantity being monitored. The low impedance to ground signal is detected by the central processor which terminates the data request signal. The central processor includes a pulse width to digital converter which produces a digital signal indicative of the time duration of the data request signal and hence the value of the quantity.
The Bogacki U.S. Pat. No. 4,202,195 discloses a meter terminal unit for an automatic meter reading system. Plural meter terminal units are connected across the power lines and each unit can be separately addressed by a control unit which sends messages in digital form by RF pulses on the power lines. A transmitter at the terminal unit receives an RF carrier signal and a binary data message. In this system, the terminal unit is addressed by RF pulse modulation and the unit sends data by a fixed frequency RF oscillator as a carrier wave which is modulated by a binary signal.
The Falck U.S. Pat. No. 3,952,285 relates to a security polling system with multiple transponders on a pair of telephone lines. Each transponder is assigned an account number for addressing purposes and a selected transponder is polled by transmitting its assigned frequency from the central station. The transponder with the transmitted account number is activated to send its status condition to the central station. In this system, each transponders requires two different fixed frequency oscillators and the status information is represented by the duration and sequency of frequency bursts from the oscillators.
The Augenblick et al U.S. Pat. No. 3,944,928 discloses a central transmitter which radiates a carrier signal having an address encoded thereon. The receiving stations have an address decoder and each station includes a rectifier for the received carrier signal to provide energization of the station. A data signal is used to modulate a harmonic of the carrier signal which is radiated by an antenna back to the central transmitter.